US-based watchdog International Rivers accuses NTPC of violating an agreement not to start commercial operations until local residents had been compensated for damage to their land.
More than 6,000 people were resettled to make way for the 1,070-megawatt dam.
According to International Rivers, they are now living on very poor land which makes it difficult to farm.
"They are actually happy with the new houses, [...] the health centre, the education, but all of them are saying they have a difficult time to find food," Ikuko Matsumoto, the group's Lao programme director, told RFI.
Matsumoto says she visited the villagers recently and saw no evidence of the irrigation NTPC had promised to provide to improve the soil.
"There is a concrete tank sitting in the middle of the field, but the villagers can't use the water yet."
In contrast the villagers' vegetable gardens, planted the length of the river, have been flooded after the dam pushed water levels 3.6 metres higher than usual.
The relocated residents have not yet received any compensation from NTPC, according to Matsumoto.
"How the company is going to compensate for these people is not really clear, and the villagers don't know when and how much," she says.
International Rivers is demanding an immediate suspension of the dam's operations until NTPC "complies with its legal agreements" to compensate villagers.
But the World Bank, which has supervised and monitored the project, denied the activists' allegations.
"The notion that the project is in violation of legal agreements is incorrect," the bank said in a statement to AFP.
NTPC, which is co-owned by the government of Laos and electricity companies from France and Thailand, says it is working on a range of social and environmental programmes and that independent monitors are checking progress to ensure it meets its obligations.